1. Field of the Invention
This invention relates to an ophthalmologic instrument which includes an alignment optical system for detecting the alignment of a principal optical system of the instrument with a subject's eye.
2. Description of the Prior Art
Heretofore, an ophthalmologic instrument is known which includes an alignment optical system. In the alignment optical system, a beam of light for alignment emitted by a light source for alignment is guided to a subject's eye. The alignment light is reflected by the eye and then received by a light receiving means. Based on a state of the light received by the light receiving means, the alignment optical system detects the alignment of the principal optical system of the instrument with the eye.
A description will be given of a case where such an alignment optical system is applied to a non-contact type of tonometer as an ophthalmologic instrument. In this case, a light source to be used emits infrared rays of light. A beam of light for alignment emitted by the light source is projected onto a cornea of the eye through a jet nozzle. The alignment beam of light is reflected by the cornea and then received by the light receiving means. From a quantity of the reflected light received by the light receiving means or a point where the reflected light is received by the same, the light receiving means detects the alignment of a visual line of the eye with an optical axis of the principal optical system.
In the non-contact type of tonometer, a working distance between the eye and the jet nozzle is made short (e.g., 11 mm) enough to attain measurement accuracy.
However, such a short distance raises a problem of stains. That is, tears, fine dust, and so on are scattered from the eye by blowing air pulses from the jet nozzle toward the eye in order to make the cornea flat. Therefore, a case occurs in which part of the scattered particles adhere to optical members, such as an objective lens or the like, of the instrument and, if it is neglected to remove the part of the particles that will be a stain, the quantity of the reflected light is greatly lessened by the stain.
The decrease forms a cause of wrong operations of the instrument. The reason is that, if a beam of light different from the reflected light is received by the light receiving means at the same time together with the reflected light, a judgment on which is the alignment light is formed based on a quantity of light received by the light receiving means. Therefore, a case occurs in which, although alignment is carried out in fact, the instrument judges that the alignment is not correctly done.